The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.
The present invention relates to composite structures and to methods and apparatuses pertaining to same, more particularly wherein the composite structures are to some degree or in some respect characterized by lamination.
Many composite structures include layers which are bonded together. Various applications have given rise to concern about delamination resistance at either or both of primary bond sites and secondary bond sites. The term xe2x80x9cdelamination resistancexe2x80x9d is conventionally understood to encompass xe2x80x9cstrengthxe2x80x9d (e.g., through-thickness tensile strength) and/or xe2x80x9ctoughnessxe2x80x9d (e.g., Mode I fracture toughness). The terms xe2x80x9cthrough-thickness strength,xe2x80x9d xe2x80x9cout-of-plane strengthxe2x80x9d and xe2x80x9cinterlaminar strengthxe2x80x9d are synonymous in conventional usage.
Improvement of the delamination resistance of composite laminates has been attempted through a variety of mechanisms. Among the known mechanical methodologies for increasing delamination resistance are the following: (i) the insertion of metal pins, stitches or fibrous rods through the thickness of the composite laminate; and, (ii) the alteration of the style of reinforcement, e.g., through utilization of tufted fabrics to improve adhesion. There are drawbacks associated with these mechanical methodologies, such as cost, degradation of mechanical properties in the plane of the laminate, etc. Another conventional methodology for enhancing delamination resistance involves toughening of brittle resins with particles made of rubber (or another high elongation material); according to these approaches, toughness is generally achieved at the expense of strength.
It is often desirable to improve both strength and toughness, for the ability to do so could delay both crack initiation and crack propagation in composite laminates. Furthermore, any improvements in through-thickness strengths in composite laminates can be viewed as advantageous, since their low strengths in that direction are usually the limiting factor in design of structures with composites. Moreover, through-thickness strength is normally very sensitive to quality; thus, improvements in toughness could minimize the flaw sensitivity of the through-thickness strength. This is significant particularly because through-thickness stresses tend to arise in structural details which are difficult to fabricate at the level of quality of flat panels.
Composite structural details for U.S. Navy marine applications frequently require the use of secondary bonds for fabrication in a shipyard environment. Secondary bond sites are interfaces where there has been lamination over a cured laminate, and they can represent a weak link in composite laminate performance. The typical microstructural appearance of a secondary bond is a discrete, linear resin-rich region between the layers of a composite laminate. This resin-rich region can result in a composite laminate with reduced strengths through-the-thickness of the laminate (i.e., normal to the secondary bond) and reduced resistance to delamination.
In view of the foregoing, it is an object of the present invention to provide a composite structure, and method and apparatus for fabricating same, wherein the composite structure has superior performance in terms of delamination resistance.
It is a further object of the present invention to provide such composite structure, method and apparatus wherein the delamination resistance includes either or both of through-thickness strength and fracture toughness.
Another object of this invention is to provide such composite structure, method and apparatus wherein the improvement of delamination resistance with respect to toughness does not result in the worsening of delamination resistance with respect to strength, or vice versa.
A further object of this invention is to provide such composite structure, method and apparatus wherein the improvement of delamination resistance does not result in the worsening of a mechanical property unrelated to delamination resistance.
Another object of this invention is to provide such composite structure, method and apparatus which are cost-effective.
The present invention features the effectuation of a fractal form of disordered geometry at a composite lamina interface. Fractal geometry is advantageous (vis-a-vis"" non-fractally disordered geometry) because it represents a reproducible and simplified mathematical regime for introducing geometric disorder. The disordered interface geometryxe2x80x94and specific characteristics associated therewithxe2x80x94are inventively related to specific mechanical or material properties such as through-thickness strength and fracture toughness. According to this invention, fractal topology is not only related to certain material/mechanical properties, but is also used to selectively enhance particular material/mechanical properties. In particular, fractal interfaces in composite laminates are inventively used as strengthening and/or toughening mechanisms.
In accordance with this invention, a composite structure comprises a first lamina and a second lamina. The first lamina has a first laminal surface which defines a first laminal fractal profile. The second lamina has a second laminal surface which defines a second laminal fractal profile. The second laminal fractal profile is complementary with respect to the first laminal fractal profile. The first laminal surface and the second laminal surface join so as to form an interface which defines an interfacial fractal profile. The interfacial fractal profile is described by the engagement of the first laminal fractal profile and the second laminal fractal profile.
Also in accordance with this invention, a method for making a composite structure comprises: providing a metal mold; resin transfer molding a first lamina; and, resin transfer molding a second lamina. The metal mold has a mold surface which defines a mold fractal profile. The first lamina has a first laminal surface which defines a first laminal fractal profile which is effected by the mold fractal profile. The second lamina has a second laminal surface which defines a second laminal fractal profile which is effected by the first laminal fractal profile.
The present invention admits of embodiments wherein there is secondary bonding of the first lamina and the second lamina, as well as embodiments wherein the first lamina and the second lamina are joined in the absence of secondary bonding. When secondary bonding is implemented, the inventive composite structure comprises a secondary bond layer which at least substantially occupies the fractally profiled interface between the first lamina and the second lamina. The inventive fabrication method can thus include secondarily bonding the second laminal surface with respect to the first laminal surface, in association with the resin transfer molding of the second lamina.
The following papers, hereby incorporated herein by reference, disclose relationships between various forms of microstructural disorder and improvement in various macroscopic properties:
Chen, Z. and Mecholsky, Jr., J. J. September 1993. xe2x80x9cControl of Strength and Toughness of Ceramic/Metal Laminates Using Interface Design.xe2x80x9d Journal of Materials Research 8(9):2362-2369;
Tancrez, Jean-Pierre, Pabiot, Jose and Rietsch, Francois. 1996. xe2x80x9cDamage and Fracture Mechanisms in Termoplastic-Matrix Composites in Relation to Processing and Structural Parameters.xe2x80x9d Composites Science and Technology 56:725-731;
Zumbrunnen, D. A. 1997. xe2x80x9cMicrostructures and Physical Properties of Composite Materials Evolved from Chaos.xe2x80x9d Proceedings of the Fourth Experimental Chaos Conference, Aug. 6-8, 1997, Boca Raton, Fla. Tancrez et al. disclose improved ductility in toughened polymers where small, non-propagating crases formed a stable xe2x80x9cmicronetxe2x80x9d through which a dominant crack would have to propagate.
Zumbrunnen discloses use of chaotic motion to develop very fine-scale microstructures and interfaces in two-phase thermoplastic blends. According to Zumbrunnen, progressive intertwining of the major and minor phase components led to enhanced material properties (toughness, ductility, strength and electrical conductivity).
Chen et al. disclose in-plane loading of a composite laminate comprising a brittle alumina layer and a ductile nickel layer. According to Chen et al., the greater the tortuosity (tortuosity quantified by fractals) of the interface between alumina and nickel layers in ceramic/metal composites, the greater the force required to separate the layers. Chen et al. found that there was an increase in strength, but a decrease in toughness, with increasing fractal dimension (i.e., increasing disorder). Chen et al. speculate that the decrease in toughness which they observed resulted from the inability of the ductile layer to plastically deform as it was constrained by the brittle layer.
As contrasted with Chen et al., Tancrez et al. and Zumbrunnen et al., the present invention uniquely concerns the relationship of a disordered interfacial microstructure of a composite laminate to two specific macroscopically improved properties, viz., out-of-plane strength and fracture toughness.
It is noted that the analysis and testing performed by the inventors has involved out-of-plane loading, whereas Chen et al. discloses in-plane loading. Chen et al. not only used a different loading direction but also used different materials. Moreover, Chen et al. addressed a known phenomenon which is a manufacture by-product or artifact of linearly interfaced laminates. That is, Chen et al. observed that the interface bond geometry of laminates which are essentially linear will be characterized by disorder along the border or periphery, due to irregularities in fiber packing at such border or periphery.
As distinguished from Chen et al., the present invention uniquely provides disorder (in particular, fractality) of the the entire ductile (resin) secondary bond inteface layer, not just of the border or periphery. The disorder is inventively achieved by using a machined mold plate and by carefully choosing fiber reinforcement so as to ensure that the fibers nest in the peaks and valleys of the interface. The inventive results of numerical analyses suggest that both strength and toughness may be enhanced if small cracks form but do not propagate. In other words, the present invention uniquely avails of a newly discovered relationship whereby both strength and toughness increase with increasing disorder; in particular, when the disorder is fractal in nature, both strength and toughness increase with increasing fractal dimension.
According to inventive principles, a fractal interface geometry provides benefits over a more ordered interface geometry through the reduction of pressure stresses and the introduction of yield stress gradients. The formation of the small cracks results in the release of constraints to plastic flow. However, the cracks do not propagate, due to the tortuosity of the crack path and to the complex local stress state, both of which are introduced by the disordered geometry. The small cracks initiate at sites of localized tensile stress concentration, generally situated proximate the xe2x80x9cmaximaxe2x80x9d and xe2x80x9cminimaxe2x80x9d of the fractal interface profile. These small cracks act not only as liberators of transverse constraints to plastic flow, but also as energy-absorbing mechanisms.
The present inventorship includes U.S. Navy employees. Secondary bonds appear frequently in Navy composite structures due to thickness, geometry and fabrication constraints. Secondary bonds represent a potential weak link in the performance of composite structural details, because improper fabrication and assembly may result in lower strength and toughness at the secondary bond site as compared to the primary structure. Various embodiments of the present invention can be used to improve secondary bond strength/toughness or provide alternate fabrication and assembly options. This would result in improved structural performance and/or reduced productions costs.
More generally, the inventive utilization of a controlled, disordered interface, geometry to improve composite strength and toughness in the presence of through-thickness stresses opens up additional options, beyond material selection, to improve composite structural performance and efficiency.
U.S. Pat. No. 6,333,092, of which this application is a division, includes an xe2x80x9cAppendix Axe2x80x9d which is a copy of a thirty-six page manuscript, authored by joint inventors Dale Karr and Karin Gipple, entitled xe2x80x9cFractal Fracture Mechanics of Interlaminar Tensile Failure of Composites.xe2x80x9d U.S. Pat. No. 6,333,092 is incorporated herein by reference and hence this manuscript (which is included in U. S. Pat. No. 6,333,092) is incorporated herein by reference. This manuscript was submitted to the International Journal of Fracture, but has not as yet been accepted for future publication. Previously, a similar rendering of this manuscript was submitted to, but not accepted for future publication by, Mechanics of Materials. 
Other objects, advantages and features of this invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.